This invention relates to an X-ray generator and in particular to an X-ray generator suitable to be closely coupled to a focusing X-ray device.
X-ray generators comprise an electron gun, an X-ray target and an X-ray exit window, generally in a sealed evacuated X-ray tube. Prior art generators produce X-ray beams having a relatively large focal spot or line. Many applications require a precisely collimated X-ray beam. To achieve this relatively small apertures are coupled with the generator to restrict beam diameter and divergence, but this results in a large loss of X-ray intensity.
For many applications the most effective way of using the X-rays emitted from the target of an X-ray generator is to form an image of the source, i.e. of the electron focus on the target, on the specimen. For crystallographic applications, it is normally essential that the convergence or divergence of the rays incident on the sample be very small. To maximise the X-ray intensity at the sample the angle of collection at the source should be as large as possible. The combination of these two requirements implies that the imaging optics should magnify. The sample size determines the maximum useful image size (see FIG. 3). FIG. 3 shows that the ratio of the collecting angle a at the source S to the beam convergence angle .beta. at the image I is equal to the magnification of the focusing collimator or focusing mirror F. In single-crystal diffractometry, for example, the specimen crystal is frequently about 300 .mu.m in diameter. The X-ray source should, therefore, be much smaller than 300 .mu.m.
Maximum power loading of the target, without damage to its surface is greatest when the source is a line focus at a small take-off angle to give a foreshortening of about 10 times.
It is an object of the present invention to provide an X-ray generator which produces an X-ray source having a focal spot or line of very small dimensions. It is a further object of the present invention to provide an X-ray generator capable of producing a high intensity X-ray beam at a relatively small point of application using a low operating power.